Designer Flavoproteins for the Elucidation of the Fundamental Mechanism of Potential Inversion
Flavins are arguably one of the most versatile cofactors in biology, resulting from their ability to facilitate both 1e- and 2e- transfer reactions, as well as their widely-varying reduction potentials that can be tuned by the local protein environment. Flavins have three redox states: oxidized (OX), 1e--reduced semiquinone (SQ), and 2e--reduced hydroquinone (HQ). Their reactivity in redox processes is determined by E’OX/SQ and E’SQ/HQ, which is affected by electrostatic and H-bonding interactions in the flavin binding site. Free in solution, flavins exhibit “inverted potentials”, where the 1e– reduced state is more unstable than the fully-reduced state. The flavin binding site of flavodoxins, a class of electron transfer flavoproteins, alters the 1e- reduction potentials such that they become “normally” ordered, stabilizing E’OX/SQ relative to E’SQ/HQ. In other cases, the protein environment can increase potential inversion. A dramatic example can be found in flavin-based electron bifurcation (FBEB), which utilizes significant potential separation (ΔE) to conserve energy from exergonic redox reactions by concomitantly driving endergonic ET. The properties of the flavin binding site that induce this inversion are unknown, largely due to the limited mutagenesis space in authentic EB-ases and lack of physical methods for measuring highly-inverted 1e- reduction potentials. Since potential separation is fundamental to the mechanism of energy conservation in FBEB, understanding its cause is both interesting from a basic science perspective, and valuable to the field of electrocatalysis. To move towards this goal, it is essential to develop minimalist models for potential inversion, since the study of this phenomenon in native EB-ases is nontrivial. We have demonstrated that iLOV can be used as a model flavoprotein to evidence the effects of near-flavin mutations on redox behavior and potential inversion, substituting Lys to illustrate the influence of an ionizable and H-bonding functional group on semiquinone stability. We have designed expression and reconstitution methods to recombinantly produce iLOV WT, Q104K, and Q104A in high holoprotein yield. The 2e- reduction potential of iLOV WT and Q104K was measured by protein film voltammetry, and equilibration studies with a redox dye were conducted to confirm that protein-film electrochemistry corresponded to bulk properties. The semiquinone protonation state was characterized by EPR and UV-vis. Using spectral data to infer concentrations of the three flavin redox states at electrochemical equilibrium, ΔE was calculated for iLOV WT, iLOV Q104K and iLOV Q104A. The results suggest that iLOV WT has the most inverted 1e- potentials, followed by Q104A, with Q104K having the least potential separation. The substitution of Ala at the N5 position appeared to stabilize the SQ, although this is likely due to exposure to ambient light. The addition of Lys at N5 appears to influence the thermodynamic properties of the flavin such that E’OX/SQ, and seemingly E’SQ/HQ, is more positive. This can be rationalized by its relative acidity and closer pKa matching between the flavin and Lys, as compared to Gln in iLOV WT. Our discoveries on the crucial elements of potential inversion in flavin redox chemistry may shed light on the mechanisms of potential tuning in natural systems, particularly extreme potential inversion of EB-ases.